The present invention relates to a solar cell substrate, thin-film solar cell, and multi-junction thin-film solar cell, which are capable of inexpensively providing solar cells having stable and high photo-electric conversion efficiency.
Fossil fuel such as petroleum is expected to be in short of supply in the future, and has the problem of carbon dioxide emission, in addition to causing the global warming effect. In recent years, solar cells have been under the focus of attention as a replacement energy source for fossil fuel such as petroleum.
The solar cells incorporate semiconductors with the p-n junction in a photo-electric conversion layer which converts light energy into electricity. The most common semiconductor with the p-n junction is silicon. In view of photo-electric conversion efficiency, mono-crystalline silicon is preferably used for the semiconductor. However, semiconductors with mono-crystalline silicon have problems in that it is difficult to supply the raw material, to increase its area, and to reduce cost.
Meanwhile, a thin-film solar cell which incorporates amorphous silicon as the photo-electric conversion layer has been put in practical applications to increase area and reduce cost. Further, use of crystalline silicon as the photo-electric conversion layer has been investigated in an effort to realize a solar cell with high and stable photo-electric conversion efficiency which can match up against that of the mono-crystalline silicon solar cell, and with a large area and low cost comparative to that attained by the amorphous silicon solar cell. A particular interest has been on a thin-film solar cell with a crystalline silicon thin film (xe2x80x9ccrystalline silicon thin-film solar cellxe2x80x9d hereinafter), which is formed by the thin-film deposition technique employing the chemical vapor deposition method (xe2x80x9cCVD methodxe2x80x9d hereinafter) which is used in forming the amorphous silicon.
Japanese Unexamined Patent Publication No. 289173/1989 (Tokukaihei 1-289173) (published date: Nov. 21, 1989) discloses a multi-junction thin-film solar cell which is formed by depositing a photo-electric conversion element having an amorphous silicon active layer as an active layer, and another photo-electric conversion element having an active layer of crystalline silicon which has a comparatively smaller energy gap than the amorphous silicon. This multi-junction thin-film solar cell is structured so that sun light is incident on the side of the photo-electric conversion element having the active layer of amorphous silicon, which is advantageous in utilizing the solar energy more efficiently than the mono-junction type. Further advantages of this structure are high open-circuit voltage due to the series connection of the plurality of photo-electric conversion elements, and slow degradation rate of photo-electric conversion efficiency which degrades due to the Staebler-Wronski effect. This slow degradation rate of photo-electric conversion efficiency is made possible by the thin amorphous silicon layer as the active layer. Another advantage of this structure is that it allows the amorphous silicon layer and the crystalline silicon layer to be fabricated using the same device, which has made this structure a target of active research and development to attain high efficiency and low cost at the same time.
Note that, in the following description of the present invention, the term xe2x80x9ccrystallinexe2x80x9d is meant to indicate not only a crystalline state of essentially pure crystals such as xe2x80x9cmono-crystalxe2x80x9d or xe2x80x9cpoly-crystalxe2x80x9d, but also a mixed state of crystal component and amorphous component, which state is referred to as xe2x80x9cmicrocrystalxe2x80x9d.
One of the important techniques which is an important factor in realizing a thin-film solar cell with high photo-electric conversion efficiency is light-trapping. The light-trapping is the phenomenon in which the quantity of light absorbed in the photo-electric conversion layer is increased by increasing the optical path length, which is attained by forming irregularities on the surface of the transparent conductive film or metal layer in contact with the photo-electric conversion layer, so as to cause light to scatter at the interface.
For example, Japanese Patent No. 1681183 (published date: Apr. 6, 1983) and No. 2862174 (issued Feb. 24, 1999) disclose solar cell substrates which specify particle size or size of irregularities of the transparent conductive film which is formed on the glass substrate.
The improved photo-electric conversion efficiency by the light-trapping effect enables the photo-electric conversion layer to have a thinner thickness. This effect suppresses deterioration of light caused by the Staebler-Wronski effect, in the case of the amorphous silicon solar cell.
Further, due to its light absorbing characteristics, the crystalline silicon solar cell conventionally required a thickness on the order of several microns, which is several times to several ten times greater than that required for the amorphous silicon. However, even with the crystalline silicon solar cell, a deposit time can be made much shorter when the photo-electric conversion efficiency is improved by the effect of light-trapping.
That is, light-trapping is the essential technique for realizing high efficiency, high stability, and low cost at the same time, which are all required for practical applications of the thin-film solar cell.
However, despite active research and development to this date, the photo-electric conversion efficiency of the conventional crystalline silicon thin-film solar cells has only reached the level of the photo-electric conversion efficiency of the amorphous silicon.
Technical Digest of the International PVSEC-11, Sapporo, Hokkaido, Japan, 1999 (H. Yamamoto et al.) has the following report.
Microcrystalline silicon, when deposited by the plasma CVD method on the Asahi-U substrate, which is a glass substrate with tin oxide deposited thereon to have microscopic irregularities, causes crystal grains of the silicon to grow primarily in a vertical direction with respect to each surface of the microscopic irregularities of tin oxide. The crystal grains grown in this manner from each different surface of the irregularities have different crystal directions and they collide with one another. The result is mass defects. These defects need to be suppressed to a minimum because they become a recombination center of carriers (electrons and holes) to severely degrade photo-electric conversion efficiency.
H. Yamamoto et al. has the following report as well.
The size of the irregularities was made smaller by depositing zinc oxide to a thicker thickness on the tin oxide having surface irregularities. The result was the same as that obtained using only tin oxide, causing growth of crystal grains of the silicon in a vertical direction with respect to the surface of zinc oxide and thereby causing collision of crystal grains which grow from each different surface. However, the differences of directions were smaller in this case, and less defects were incurred.
It is therefore apparent that the size of irregularities on the substrate surface should be reduced as much as possible in order to reduce defects in the crystalline silicon thin-film. While this may be the case, as noted above, light-trapping is necessary for the thin-film solar cell, and it is not entirely preferable to eliminate or reduce the surface irregularities when practical applications are at hand.
Meanwhile, the solar cell substrate with the transparent conductive film having surface irregularities, as disclosed in the foregoing Japanese Patent No. 1681183 and No. 2862174, has the problem of cost, which is one factor that prevents wide-spread use of the thin-film solar cell. One approach to solve this problem is to use zinc oxide for the transparent conductive layer. Zinc oxide is comparatively cheaper than other materials such as tin oxide or ITO which are widely used as the material of the transparent conductive film. Further, the advantage of high plasma resistance makes zinc oxide a suitable material for the transparent conductive film used for thin-film solar cells.
Examples of using zinc oxide for the transparent conductive film of the thin-film solar cell are disclosed in Japanese Patent No. 2974485 (issued Nov. 10, 1999), No. 3072832 (issued Aug. 7, 2000), and Japanese Unexamined Patent Publication No. 233800/1999 (Tokukaihei 11-233800) (published date: Aug. 27, 1999). These publications disclose thin-film solar cells with irregularities which are formed by etching a zinc oxide layer which was deposited by sputtering. However, these are all examples of optimization of amorphous silicon solar cells, and they require other modifications to be applicable to crystalline silicon thin-film solar cells. Specifically, there has been no known structure of surface irregularities of the substrate which can exhibit high light-trapping effect without causing defects in the photo-electric conversion layer.
Further, Japanese Unexamined Patent Publication No. 117006/1998 (Tokukaihei 10-117006) (published date: May 6, 1998), No. 294481/1998 (Tokukaihei 10-294481) (published date: Nov. 4, 1998), No. 214728/1999 (Tokukaihei 11-214728) (published date: Aug. 6, 1999), and No. 266027/1999 (Tokukaihei 11-266027) (published date: Sept. 28, 1999), No. 58892/2000 (Tokukai 2000-58892) disclose structures of thin-film solar cells and multi-junction thin-film solar cells.
Specifically, the solar cells disclosed in these publications have a structure in which a lower photo-electric conversion element having a photo-electric conversion layer of crystalline silicon layer is formed on a rear electrode having surface irregularities, and the foregoing publications disclose a structure of a thin-film solar cell in which the crystalline silicon layer has a primary crystal orientation plane (110) parallel to the substrate surface.
However, the structures disclosed in these publications all have an element structure of a substrate type in which light is incident on the side of the photo-electric conversion element, and even in the element structure of a superstrate type in which light is incident on the side of the substrate using a transparent substrate, there has been no known irregular structure which can be suitably used to realize low defect density and good light-trapping effect at the same time in the crystalline silicon thin-film.
It is an object of the present invention to provide a solar cell substrate which can be manufactured at low cost with sufficient light-trapping effect without increasing defect density in a crystalline semiconductor layer, and a thin-film solar cell, and a multi-junction thin-film solar cell.
In order to achieve this object, a solar cell substrate of the present invention has irregularities on the surface which is in contact with a photo-electric conversion layer, light being incident on the solar cell substrate on the side of the irregularities, wherein: a height of the irregularities is set so that a root mean square height is in a range of 15 nm to 600 nm, and tan xcex8 is in a range of 0.10 to 0.30, where xcex8 is an angle of incline of a surface of the irregularities with respect to an average line of the irregularities.
According to this arrangement, since the irregularities on the surface of the solar cell substrate are in contact with the photo-electric conversion layer, the light incident toward the irregularities is scattered at the interface. The scattering of light increases the optical path length, and thus the quantity of light absorbed in the photo-electric conversion layer. By this trapping of light, photo-electric conversion efficiency is increased. The improved photo-electric conversion efficiency enables the photo-electric conversion layer to have a thinner thickness. As a result, much less deposit time and much less manufacturing cost are required for the photo-electric conversion layer.
Incidentally, depending on such factors as the height or shape of the irregularities on the surface of the solar cell substrate, there are cases where crystal grains of a crystalline semiconductor which is formed as the photo-electric conversion layer on the irregular surface collide. This causes defects. Such defects become a recombination center of carriers and have detrimental effect on photo-conversion efficiency.
In view of this, according to the present invention, the height of the irregularities is set so that the root mean square height of the irregularities is in a range of 15 nm to 600 nm, and tan xcex8 is in a range of 0.10 to 0.30, where xcex8 is the angle of incline of the irregular surface with respect to an average line of the irregularities. This arrangement greatly reduces occurrence of crystal collisions without losing light-trapping effect. In effect, it is ensured that photo-electric conversion efficiency does not become poor due to defects.
Further, in order to achieve the foregoing object, a thin-film solar cell according to the present invention includes: a solar cell substrate having irregularities on a surface which is in contact with a photo-electric conversion layer, light being incident on the solar cell substrate on the side of the irregularities, a height of the irregularities being set so that a root mean square height is in a range of 15 nm to 600 nm, and tan xcex8 in a range of 0.10 to 0.30, where xcex8 is an angle of incline of a surface of the irregularities with respect to an average line of the irregularities, the solar cell substrate having the photo-electric conversion layer which is made up of at least one photo-electric conversion element.
According to this arrangement, the absorbed quantity of light by the light-trapping effect can be increased without causing more defects in the photo-electric conversion layer, thereby providing the solar cell substrate with stable and high photo-electric conversion efficiency at low cost.
Further, in order to achieve the foregoing object, a solar cell substrate of the present invention has irregularities on a surface which is in contact with a photo-electric conversion layer, light being incident on the solar cell substrate on the other side of the irregularities, wherein: a height of the irregularities is set so that a root mean square height is in a range of 25 nm to 600 nm, and tan xcex8 is in a range of 0.07 to 0.20, where xcex8 is an angle of incline of a surface of the irregularities with respect to an average line of the irregularities.
According to this arrangement, since the irregularities on the surface of the solar cell substrate are in contact with the photo-electric conversion layer, the light incident toward the irregularities is scattered at the interface. The scattering of light increases the optical path length, and thus the quantity of light absorbed in the photo-electric conversion layer. By this trapping of light, photo-electric conversion efficiency is improved. The improved photo-electric conversion efficiency enables the photo-electric conversion layer to have a thinner thickness. As a result, much less deposit time and much less manufacturing cost are required for the photo-electric conversion layer.
Incidentally, depending on such factors as the height or shape of the irregularities on the surface of the solar cell substrate, there are cases where crystal grains of a crystalline semiconductor which is formed as the photo-electric conversion layer on the irregular surface collide. This causes defects. Such defects become a recombination center of carriers and have detrimental effect on photo-conversion efficiency.
In view of this, according to the present invention, the height of the irregularities is set so that the root mean square height of the irregularities is in a range of 25 nm to 600 nm, and tan xcex8 is in a range of 0.07 to 0.20, where xcex8 is the angle of incline of the irregular surface with respect to an average line of the irregularities. This arrangement greatly reduces occurrence of crystal collisions without losing light-trapping effect. In effect, it is ensured that photo-electric conversion efficiency does not become poor due to defects.
Further, in order to achieve the foregoing object, a thin-film solar cell according to the present invention includes: a solar cell substrate having irregularities on a surface which is in contact with a photo-electric conversion layer, light being incident on the solar cell substrate on the side of a surface opposite the surface with the irregularities, a height of the irregularities being set so that a root mean square height is in a range of 25 nm to 600 nm, and tan xcex8 in a range of 0.07 to 0.20, where xcex8 is an angle of incline of a surface of the irregularities with respect to an average line of the irregularities, the solar cell substrate having the photo-electric conversion layer which is made up of at least one photo-electric conversion element.
According to this arrangement, the absorbed quantity of light by the light-trapping effect can be increased without causing more defects in the photo-electric conversion layer, thereby providing the solar cell substrate with stable and high photo-electric conversion efficiency at low cost.
Further, in order to achieve the foregoing object, a multi-junction thin-film solar cell of the present invention includes: a plurality of photo-electric conversion elements on the opposite side of a side of a substrate on which light is incident; and an intermediate layer, having irregular surfaces, provided on at least one of the photo-electric conversion elements adjacent to one another, a height of the irregularities of the intermediate layer being set so that a root mean square height is in a range of 25 nm to 600 nm, and tan xcex8 in a range of 0.07 to 0.20, where xcex8 is an angle of incline of a surface of the irregularities with respect to an average line of the irregularities.
According to this arrangement, the intermediate layer is provided between adjacent photo-electric conversion elements. Directly connecting adjacent photo-electric conversion elements means connecting layers of different conduction types. This causes deficiencies such as connection failure by the mixing of impurities which are generated by the connection of the opposite direction. The intermediate layer is provided to prevent such deficiencies.
The intermediate layer has irregularities at least on the surface on the other side of the surface facing the substrate. Light is scattered at the interface of the irregular surface of the intermediate layer, and the photo-electric conversion layer. The scattering of light increases the optical path length and thus the quantity of light absorbed in the photo-electric conversion layer. By this trapping of light, photo-electric conversion efficiency is improved. The improved photo-electric conversion efficiency enables the photo-electric conversion layer to have a thinner thickness. As a result, much less deposit time and much less manufacturing cost are required for the photo-electric conversion layer.